The invention more precisely relates to those devices that are equipped with Holter functions, i.e., with functions for recording over a long time period, from several days to several months, data collected by the device, typically data relating to the cardiac or ventilatory activity of the patient. Information thus recorded can be transmitted later on, by telemetry, with an ancillary device or “programmer” for display and analysis.
The physiological signals collected by these implants can be of very different natures:                signals representative of the cardiac activity and the heartbeat rate measured by means of atrial and/or ventricular implanted electrodes—these signals are usually called “electrograms,” as opposed to “electrocardiograms” collected by external electrodes;        signals representative of respiratory activity, for example, the minute ventilation signal obtained by measurement of the transthoracic impedance taken between the metal case of the device and a distant electrode;        signals representative of blood flow or myocardium contractility, obtained from an intracardiac measurement of impedance (e.g., transvalvular, between two cavities located on the same side of the heart, or trans-septum impedance, inter-ventricular or oblique impedance, taken between two cavities located on two different sides);        signals of measurement of the oxygen saturation in blood, or of measurement of the blood pH, etc.        
One will include in this concept of “signals” not only the signals themselves collected by electrodes, but also the event markers of the heartbeat rate, event counters, or signals not collected but representative of a state or an action of the implant, for example, the application of a therapy, or a commutation of operating mode, etc.
EP-A-0 653 226 and its counterpart, U.S. Pat. No. 5,513,645, commonly assigned herewith to ELA Medical, describes such an implantable device—a cardiovertor, equipped with Holter recording functions able to operate a memorization of data on several levels of detail: in the case of an event (e.g., occurrence of a disorder of the rate, application of a therapy . . . ), the device records throughout a short period more detailed information (typically, an electrogram sample of the period having preceded and/or followed the particular event), while the remainder of time it records simply a chain of markers.
In this known device, as in the other implants integrating the Holter functions of recording, almost all the memory—whose size is necessarily limited because of the constraints of miniaturization and of power consumption—is reserved for the memorizing of information concerning the many algorithms and available functions such as statistics, histograms, electrogram episodes, markers triggered on particular events, etc. It is the reason for which the above mentioned EP-A-0 653 226 and U.S. Pat. No. 5,551,645 propose memorizing the electrogram only over a very short duration, limited to the moments preceding and following the triggering of a therapy, typically the two seconds preceding and following the cardioversion or defibrillation shock.
In certain circumstances, however, it can be useful to collect and memorize a significant volume of information, dedicated to a particular function, for a long given period, for example, over 24 hours.
One of the goals of the present invention is to propose an implant equipped with Holter recording functions that can be dedicated at will to the continuous recording in real time of a particular function.
Thus, for example, for the study of disorders related to sleep apnea, it would be interesting to be able to monitor continuously and over 24 hours, or at the very least for part of the night, the entirety of the minute-ventilation signal. Also, for disorders of the heartbeat rate or evaluation of atrio-ventricular conduction, it would be interesting to be able to safeguard continuously the totality of the electrogram over 24 hours, to be able later on to analyze it in same manner as an electrocardiogram recording produced by an external device connected to surface electrodes.
The basic idea of the invention consists in envisaging an implantable device whose Holter memory can be used either in a standard way (i.e., by memorizing the usual data such as statistics, histograms, markers, etc), or in a way dedicated to the follow-up of a particular function.
In the dedicated Holter configuration, the apparatus devotes the major part of its memory to the continuous recording of the chosen signal, preserving only some bytes for general information on the other functions of the apparatus. The implanted device of the invention not only monitors the recorded signal, preferably continuously, but, moreover, analyzes this collected signal in real time.
In the example where the recorded signal is a ventilatory signal, this analysis makes it possible to identify certain events such as apnea, hypopnea, or respiratory pauses, or the particular respiratory profiles, such as profiles of the Cheyne-Stokes type. One will be able to refer in this respect to EP-A-0 970 713 and its counterpart, U.S. Pat. No. 6,574,507, EP-A-1 336 422 and its counterpart U.S. Published Application No. 2004/0063375, and EP-A-1 295 623 and its counterpart U.S. patent application Ser. No. 10/255,144, all in the name of ELA Médical.
In the example where the monitored signal is a cardiac signal, the analysis makes it possible to identify events such as an arrhythmia or a disorder of conduction. The detection of these events then triggers memorization of one or more pieces of “context information,” i.e., of parallel information, distinct from the monitored signal, which will make it possible to study later the context of occurrence of the events, in particular for the purpose of diagnosis. For example, if the monitored signal is the ventilatory signal, context information could be an electrogram episode, a signal of the peak endocardial acceleration, a signal delivered by a sensor of activity such as an accelerometer integrated into the case of the device, or any other information making it possible to document the event (apnea, hypopnea . . . ) detected on the monitored signal.